Coherent light has many useful qualities and applications. Lasers are the most common sources of coherent light, and are utilized in many industries. For example, lasers are used in industrial manufacturing for various processes including cutting, machining and welding of metallic and non-metallic materials. Lasers are also used in the telecommunications industry to generate and amplify light transmitted over optical fibers, and also in many other applications.
Certain wavelengths of light can be difficult to produce directly from lasers. This may be due to material properties, e.g., energies of the light producing electron transitions, or complexities involved with managing a particular gain medium, e.g., toxicity of gases or liquids that are used. Coherent light may be produced at these otherwise problematic wavelengths by using optical parametric devices to shift the wavelength of the output of a laser. Optical parametric devices convert light of one wavelength to light of another wavelength through the process known as three-wave interaction, in which three optical waves or fields are mixed and one or two of the three optical waves may be selectively amplified. Nonlinear crystalline materials are commonly characterized as being a particular type, i.e., either Type I or Type II, according to how the effect of double refraction or birefringence affects incident light once it enters the particular crystal. A nonlinear crystal may be referred to as a Type I crystal when an incident or “pump” wave is doubly refracted into signal and idler fields or waves that have the same polarization, which is orthogonal to the pump wave. A Type II nonlinear crystal is one producing orthogonally polarized signal and idler fields or waves from a pump wave.
Efficient heat removal from an active or nonlinear gain medium is a key issue for any high power operation, and can constrain output power scaling for a given gain medium. Excess heat, or thermal energy, within a gain medium can decrease the desired gain interaction, whether it be laser or parametric. Deleterious consequences of excessive heat in the gain medium include a reduced population inversion and thermal lensing. Thermal management issues are especially important for solid-state lasers, where, unlike gas and liquid active medium lasers, the active medium itself cannot be removed from the laser cavity to facilitate proper heat exchange.
Previous attempts have been made to improve thermal management in solid state lasers to increase power output and/or beam quality. Convective cooling has been used to remove heat from solid state gain media by having a fluid, which may be either a gas or liquid, flow over one or more surfaces of the gain particular medium. Conductive cooling methods have been used to remove heat from one or more surfaces of a solid state gain medium, typically by placing a heat sink into contact with one or more surfaces of the gain medium. Such previous convective and conductive cooling methods can be limited as to the amount of heat that can be removed from the gain medium. Because solid state gain media are typically poor heat conductors and conductors, the rate at which heat can be removed from the gain medium can be limited by the gain medium surface area that is available for heat removal.
A prior art laser in which a given volume of gain medium is separated into pieces as a way to increase the surface area available for heat transfer is described in U.S. Pat. No. 6,667,999 to Hasson et al., commonly owned by the assignee of the present application. FIG. 1 is a cross section of a prior art laser 100 as described in U.S. Pat. No. 6,667,999. The prior art laser 100 includes multiple gain cells configured in a sandwich-like arrangement along an optical axis 101 within a resonator formed by first and second mirrors 108, 110. Each gain cell consists of a disk of laser material 102, such as Nd:YAG, alternating with a disk of an optically transparent heat transfer medium (inline OTH) 104. The inline OTH 104 is described as a diamond disk. Antireflective or index matching coatings 106 are present between the disks of laser material 102 and the adjacent inline OTH 104. A peripheral OTH 114 is positioned laterally to the gain cells and in contact with a peripheral surface of the inline OTH. The peripheral OTH 114 contacts each gain cell so that heat can be transferred to a heat exchange system (not shown). When the gain cells are optically pumped to create laser gain and an optical output 116, waste heat develops in the laser gain material 102. This heat is conducted parallel to the axial direction into the inline OTH 104, e.g., diamond disks, where the heat is efficiently conducted radially by the diamond 104 to the peripheral OTH and on to the heat exchange system (not shown). A similar laser and thermal management system is described in H. P Chou, Y. Wang, V. Hasson, “Compact and Efficient DPSS Laser Using Diamond-cooled Technology”, Proc. of SPIE Conf. For HPLA V, Vol. 5448, p 550, Taos, N. Mex., April 2004.
Even though the laser 100 of FIG. 1 improves on previous techniques to remove heat from a laser gain medium, it has been observed that when pump power is increased above a certain value, the specific output for a given gain medium decreases. FIG. 2 includes FIG. 2A and FIG. 2B, which depict, respectively, a gain module 200 of the thermal management system 100 of the prior art laser of FIG. 1 at two different operational conditions. The gain module 200 includes a gain medium 202 placed between two inline OTH 204(1)-204(2) along an optical axis 201 of a resonator (not shown). The gain medium 202 is disk shaped and has first and second optical surfaces 206(1)-(2), first and second lateral sides 208(1)-(2), and a width or thickness 214. The two inline OTH 204(1)-(2) are also disk-shaped and each have first and second optical surfaces 210(1)-(4), as well as a desired thickness 212. The operational condition depicted in FIG. 2A represents a condition in which no optical pumping is present and consequently no optical output is being produced by the gain medium 202. As shown in FIG. 2A, the first and second optical surfaces 206(1)-(2) of the gain medium 202 are each initially in contact with a respective first optical surface 210(2)-(3) of the adjacent OTH 204(1)-(2).
The operational condition depicted in FIG. 2B represents one in which the gain medium 202 is producing an optical output exceeding a certain specific output value along the optical axis 201 due to incident optical pump energy of a relatively high power density. The heat generated through the optical gain process produces non-uniform thermal expansion of the gain medium 202 as the specific output of the gain medium 202 exceeds a certain value. The OTH 204(1)-(2) do not deform significantly because diamond has a very high thermal conductivity, i.e., the highest of any known substance at room temperature, as indicated by thickness 212′, which is only marginally greater than the initial thickness 212. The OTH 204(1)-(2) can consequently conduct the heat flux with a relatively low thermal gradient in the radial direction relative to the optical axis 201. Because the laser gain material 202 has a much lower thermal conductivity than the OTH, larger temperature gradients occur.
With continued reference to FIG. 2B, the temperature gradients within the gain medium 202 resulting from the optical gain process produce non-uniform expansion of the gain medium 202 in the direction of the optical axis 201. This can lead to warping of the optical surfaces 206(1)′-(2)′ as indicated by the increased thickness 214′ of the gain medium 202. The thermal expansion 214′ reaches a maximum value along the optical axis 201, as shown. Significant internal stresses may develop in the gain medium 202 if the physical movement of the OTH 204(1)-(2) and gain medium 204 are constrained relative to each other as the thermal expansion process occurs. The stresses can eventually lead to fracture of the gain material 202. The nature of the non-uniform thermal distortion is such to form the material of the initially disk-shaped gain medium into a biconvex, lens-like shape, as shown. The thermal expansion can also cause separation of the gain medium 202 and the OTH 204(1)-(2) at their interface, as shown in FIG. 2B. Separation of the gain medium 202 from the diamond OTH leads to reduced beam quality and reduced specific output for the gain medium 202, even as pump energies increase beyond a certain value.
What is desirable, therefore, is to provide apparatus, systems, and methods for thermal management that allow laser and/or parametric gain media to operate above present limits of specific output for a given solid state gain medium.